My son's ESA has been cut

Bit of a long story my son who is disabled had been getting esa 254.00 every two weeks he then went for his assessment who said he was fit for work I then sent in a mandatory reconsideration letter and they upheld the assessors decision I then sent off to the tribunal but while waiting for them I received a letter from deep reversing their original decision and giving him back his esa in the support group he had been without money for 10 weeks so by calculations they owed him 1270.00 but he got his back dated pay and they only  paid him 871.00 with no explanation on why it was only that because that works out at just over 174.00 every 2 weeks it looks like they have cut his money  my question is are they allowed to do that.